1. Field of the Invention
The present invention relates to a wavelength selector, and in particular to a wavelength selector having a switched filtering member.
2. Description of the Related Art
As multiple forms of communication traffic continue to place an increasingly heavy burden on optical networks, innovative ways of pushing more data through existing fibers is being sought. Wave division multiplexing technology (WDM) provides a method for doubling the traffic capacity of a single optical fiber, without any additional fiber. Dense wavelength division multiplexing (DWDM) offers a viable alternative for increasing the transmission capabilities of fiber-optic networks. DWDM allows multiple streams of data to share a common communication channel by separating data streams into different wavelengths. The result is a dramatic increase in the amount of bandwidth provided by a single optical fiber. DWDM increases the channel density up to 40 separate optical wavelengths, thus significantly increasing the net fiber capacity.
For an optical network using DWDM, it is necessary to route and switch optical signals based on their wavelength to optimize data traffic usage. Dropping signal from and adding signals back to the optical network is a rudimentary building block for an optical network architecture. Devices which provide such functionality in conventional optical networks are called Add-Drop modules (ADMs).
The conventional wavelength selector typically has internally disposed switches. When a node fails, the switches are toggled to establish an express path connecting the input port and the output port of the node. Thus, the incoming light stream can flow through the node without interruption.
In FIGS. 1A and 1B, the conventional wavelength selector 100 having a switch 160 and an Add-Drop module is connected to an optical network, forming a node thereof, through four ports, which are named as an input port 101, output port 102, add port 103, and drop port 104. The conventional Add-Drop module includes the add port 103, the drop port 104, a connecting path 105 and two Add-Drop filers 140 and 150, which have a wavelength selectivity of a predetermined wavelength λ1 or λ1′.
The switch 160 of the conventional wavelength selector 100 can alter the light path of inserted light streams. When the wavelength selector 100 functions properly, or is in a “Bar” state, the switch 160 connects the path 161 and the path 162. The input port 101 and the output port 102 are individually connected to the Add-Drop filters 140 and 150, as shown in FIG. 1A. Four wavelengths λ1, λ2, λ3, and λ4 pass through the input port 101 and arrive at the filter 140 through the path 161. A predetermined wavelength λ1 passes through the filter and enters the drop port 104 and forms a dropped signal. The remaining wavelengths λ2, λ3 and λ4 are reflected by the filter 140, passing through the connecting path 105 and are again reflected by the filter 150. The three wavelengths λ2, λ3, λ4 are then directed to the output port through the path 162. Meanwhile, a new data signal in the same wavelength λ1′ transmitted from the add port 103 is added back to the network through the add port 103 and filter 150 and is introduced at output port 102.
When the wavelength selector 100 fails, or is in a “Cross” state, the switch 160 connects the path 163 and the path 164. The input port 101 is connected to the output port 102. The four wavelengths λ1, λ2, λ3, and λ4 are sent to the path 163 and return directly to the output port. The data signal of the wavelength λ1′ added from the add port passes through the filter 150, enters the path 164, and is directed to the drop port through the filter 140. In the “Cross” state, the conventional wavelength selector can maintain the data transmission between the input and output port 101 and 102 without interruption caused by Add-Drop module failure from the.
U.S. Pat. No. 6,192,174 discloses a wavelength selector, which integrates the Add-Drop module with the optical switch. In FIGS. 2A and 2B, four channels are employed in the wavelength selector 200, input channels 201, 203, and output channels 202, 204. Each channel may include an optical fiber, with two filters individually disposed on the collimators of the input channel 203 and the output channel 204. When the wavelength selector 200 is in the “Bar” state as shown in FIG. 2B and four wavelengths λ1, λ2, λ3 and λ4 carried along the input port 201 arrive at the filter 240, the filter 240 passes the wavelength λ1, which is directed to the output port 204 through the collimator 214. The three remaining wavelengths λ2, λ3, and ±4 are reflected into a connecting channel 205 of the dual-fiber collimator 211 by the filter 240, entering another the dual-fiber collimator 212. The collimated beam is reflected again by the filter 250, re-enters the collimator 212, and is directed to the output port 202. At the same time, the data stream in the same wavelength λ1′ carried by the add port 203 passes through the collimator 213 and the filter 250, directed to the output port 202 through the collimator 212.
In FIG. 2B, when the wavelength selector 200 is in a “Cross” state, a prism 260 is inserted between the collimators 211˜214 to redirect the optical paths. All of the wavelengths λ1, λ2, λ3 and λ4 pass through the input port 201, the collimator 211, the prism 260 and the collimator 212, and returns directly to the network through the output port 202. Meanwhile, the wavelength λ1′ added from the add port 203 passes through the filter 250, the prism 260 and the filter 240, and is output from the drop port 204. Thus, this wavelength selector 200 completes the add-drop functions of a conventional Add-Drop module.
U.S. Pat. No. 6,192,174 discloses another wavelength selector as shown in FIGS. 3A and 3B. Four single-fiber collimators are aligned with each other. A switch member having a mirror and a filter with a wavelength selecting mechanism for predetermined wavelengths. In FIG. 3A, when the wavelength selector 300 is in the “Bar” state, four wavelengths λ1, λ2, λ3 and λ4 carried along the input port 301 arrive at the filter 342 on the switch member 340, the filter 342 passes the wavelength λ1, which is directed to the drop port 304 through the collimator 314. The three remaining wavelengths λ2, λ3, and λ4 are reflected into the collimator 312 and directed to the output port 302. Meanwhile, the data stream in the same wavelength λ1′ carried by the add port 303 passes through the collimator 313 and the filter 342 on the switch, and is directed to the output port 302 through the collimator 312.
In FIG. 3B, when the wavelength selector 300 is in a “Cross” state, all of the wavelengths λ1, λ2, λ3 and λ4 that pass through the input port 201 are reflected by the mirror 341 into the collimator 312 and directed to the output port 302. At the same time, the wavelength λ1′ added from the add port 303 is reflected by the mirror 341 and is output from the drop port 304.
In FIG. 3A, the wavelength selector 300 replaces the prism 260 in FIG. 2A with a switch member 340 having a wavelength selecting filter 342 and a mirror 341. The quality of the “Bar” State wavelength isolation for λ1 of the data streams output from the output port 302 is worse than that of the wavelength selector 200 in FIG. 2A because the data streams of the remaining wavelengths λ2, λ3 and λ4 only pass through the wavelength-selecting filter once. The residual radiation of the wavelength λ1 may jam the added data stream and cause errors. To ensure quality, the filter must have higher wavelength isolation property, which may greatly increase the fabrication cost of the wavelength selector.